At the present time, solvent-dominated recovery processes (SDRPs) are rarely used to produce highly viscous oil. Highly viscous oils are produced primarily using thermal methods in which heat, typically in the form of steam, is added to the reservoir. Cyclic solvent-dominated recovery processes (CSDRPs) are a subset of SDRPs. A CSDRP is typically, but not necessarily, a non-thermal recovery method that uses a solvent to mobilize viscous oil by cycles of injection and production. Solvent-dominated means that the injectant comprises greater than 50% by mass of solvent or that greater than 50% of the produced oil's viscosity reduction is obtained by chemical solvation rather than by thermal means. One possible laboratory method for roughly comparing the relative contribution of heat and dilution to the viscosity reduction obtained in a proposed oil recovery process is to compare the viscosity obtained by diluting an oil sample with a solvent to the viscosity reduction obtained by heating the sample.
In a CSDRP, a viscosity-reducing solvent is injected through a well into a subterranean viscous-oil reservoir, causing the pressure to increase. Next, the pressure is lowered and reduced-viscosity oil is produced to the surface through the same well through which the solvent was injected. Multiple cycles of injection and production are used. In some instances, a well may not undergo cycles of injection and production, but only cycles of injection or only cycles of production.
CSDRPs may be particularly attractive for thinner or lower-oil-saturation reservoirs. In such reservoirs, thermal methods utilizing heat to reduce viscous oil viscosity may be inefficient due to excessive heat loss to the overburden and/or underburden and/or reservoir with low oil content.
References describing specific CSDRPs include: Canadian Patent No. 2,349,234 (Lim et al.); G. B. Lim et al., “Three-dimensional Scaled Physical Modeling of Solvent Vapour Extraction of Cold Lake Bitumen”, The Journal of Canadian Petroleum Technology, 35(4), pp. 32-40, April 1996; G. B. Lim et al., “Cyclic Stimulation of Cold Lake Oil Sand with Supercritical Ethane”, SPE Paper 30298, 1995; U.S. Pat. No. 3,954,141 (Allen et al.); and M. Feali et al., “Feasibility Study of the Cyclic VAPEX Process for Low Permeable Carbonate Systems”, International Petroleum Technology Conference Paper 12833, 2008.
The family of processes within the Lim et al. references describes embodiments of a particular SDRP that is also a cyclic solvent-dominated recovery process (CSDRP). These processes relate to the recovery of heavy oil and bitumen from subterranean reservoirs using cyclic injection of a solvent in the liquid state which vaporizes upon production. The family of processes within the Lim et al. references may be referred to as CSP™ processes.
Turning away from solvent-dominated recovery processes for the moment, a background of emulsions will now be provided. Emulsions are mixtures where one fluid phase is dispersed in another. The emulsions typically comprise two immiscible phases. The two immiscible phases include a continuous (or external) phase and a discontinuous (or internal) phase. Oil-water emulsions and water-gas emulsions are the most common types of emulsions. Oil-water emulsions may be either oil-in-water emulsions or water-in oil-emulsions. Emulsions may be stable for long periods of time or be unstable and relatively rapidly separate into stratified phases. Addition of surface active agents may stabilize an emulsion, as the surface-active agent resides at the two-phase interface, reducing surface energy, and providing stability. Foams are special forms of emulsions, where the internal phase is a gas phase and a liquid is the external phase (i.e., the continuous phase). The liquid may be either oil or an aqueous fluid. Foams are also stabilized by the addition of surface-active agents. A three-phase emulsion may also be formed among oil, water, and gas. Also, surface active solids, such as certain clays, may be added to stabilize emulsions.
Some emulsions are thermodynamically stable. Such emulsions are also referred to as “microemulsions”, since the internal phase droplets may be very small. Alternatively, so-called “macroemulsions” are not thermodynamically stable and, given sufficient time, will segregate. Such emulsions are said to be kinetically stable. Nevertheless, certain macroemulsions may take months or years to significantly segregate, especially if surface active agents are present. Despite the name, the droplets in a “macroemulsion” may be fairly small, e.g. 0.1-10 microns in diameter. In the following discussion, the term “emulsion” is understood to mean a macroemulsion if not specified as a microemulsion or a thermodynamically-stable emulsion.
Emulsions have been used in oil recovery processes for multiple purposes. In some applications, surface-active agents (i.e., surfactants) have been added to a waterflood operation to form an oil-water emulsion. This is commonly known as a surfactant-flood. A surfactant-flood may be followed by injection of water viscosified with dissolved polymer to provide a more stable displacement front. This process is known as surfactant-polymer flood. In some processes, an alkali may be added to the surfactant to reduce surfactant adsorption, or to generate in situ surfactant by reaction with the oil. This process is called an alkali-surfactant-polymer flood. (“ASP flood”). In all of the above mentioned applications, the surfactant reduces the oil-water interfacial tension forming an emulsion, and this leads to additional oil recovery. Combined chemical agents can produce synergetic action, which not only reduces the amount of the chemical agents used, but also results in higher oil recovery than that obtained by a single chemical drive or two-component combined drive.
Foams in the oil industry are generally used in three different applications. The first application is for blocking the breakthrough of water or gas that is being used as a secondary oil recovery technique by pushing the oil to a receiving well. The foam preferentially flows to zones of relatively higher permeability in the formation and acts to decrease the permeation of the higher permeability zones, in order to block the breakthrough. The second application is for using the foam itself as an agent to push oil to a receiving well in secondary oil recovery. The third application is the use of foams in low density drilling muds to aid in removal of drilling debris.
Certain surfactants, such as certain fluorinated surfactants, can be used for these applications because they can efficiently and effectively foam both water and oil with gas without promoting the formation of liquid-liquid emulsions. Since these surfactants do not participate in liquid-liquid emulsification, less surfactant can be used. Certain fluorinated surfactants are also preferred surfactants in these applications because they remain surface active under the harsh conditions experienced in an oil formation, e.g., high temperature/pressure, high electrolyte concentrations, etc.
The foams produced from using fluorinated surfactants are very stable. However, this stability can be an issue once the foams are recovered on the surface, since it is desirable to break these foams for processing of the produced oils. Another undesirable characteristic of surfactants is that they can leave residue behind on the formation.
As discussed above, a viscosity-reducing solvent is applied to in situ viscous oil to reduce its viscosity and thus can provide a non-thermal mechanism to improve the mobility of the viscous oil. Hydrocarbon solvents include light hydrocarbons such as ethane, propane, or butane or liquid solvents such as pipeline diluents, natural condensate streams, or fractions of synthetic crudes. The diluent can be added to steam and flashed to a vapor state or be maintained as a liquid at elevated temperature and pressure, depending on the particular diluent composition. While in contact with the bitumen, the saturated solvent vapor dissolves into the bitumen.
U.S. Pat. No. 5,350,014 discloses a method for producing heavy oil or bitumen from a formation undergoing thermal recovery. That patent describes a method for producing oil or bitumen in the form of oil-in-water emulsions by carefully maintaining the temperature profile of the swept zone above a minimum temperature.
Further, U.S. Pat. Nos. 5,060,727, 5,027,898, 4,540,050, 4,513,819, 4,444,261, 4,280,559, 5,855,243 and 5,910,467 disclose methods of viscous oil recovery using liquid-liquid or liquid-gas emulsions.
U.S. Pat. No. 3,342,256 claims the recovery of oil from subterranean oil-bearing formations wherein CO2 is introduced into the formation and then driven through the formation from an injection well to a recovery well by means of an aqueous drive liquid, the improvement which comprises disposing a surfactant solution, capable of forming a stable foam under formation conditions, in the formation not later than the introduction of the CO2, and prior to the driving of the CO2 by means of the aqueous drive liquid.
U.S. Pat. No. 4,495,995 claims a process for temporarily plugging permeable portions of a subterranean formation which comprises driving a composition formed by interacting aqueous surfactant solution and CO2 in the form of a dense fluid or a liquid into the permeable portions of the underground formation wherein the subterranean formation is at a pressure in a range of about 700 to about 5000 psi and a temperature in a range of about 50° to about 200° F. The patent also claims introducing into the subterranean formation a drive fluid, and producing recovered oil and drive fluid from at least one additional well penetrating the subterranean formation.
U.S. Pat. No. 4,706,752 claims a method for reducing the permeability of higher permeability zones of an oil bearing subterranean reservoir having heterogeneous permeability and being penetrated by at least one well, the method comprising injecting through a well and into the reservoir an aqueous liquid solution of a water soluble surface active agent; a foam emplacement gas mixture consisting essentially of carbon dioxide and a crude oil-insoluble, noncondensable, non-hydrocarbon gas, the injection being under conditions such that the gas mixture maintains a density between 0.01 and 0.42 grams per centimeter in the reservoir; allowing stable foam to form in the higher permeability zones; diverting subsequently injected gases into lower permeability zones of the reservoir without destroying the stable foam; and producing oil from the reservoir. Similarly, U.S. Pat. No. 5,105,884 claims a process for improving sweep efficiency in subterranean oil-bearing formations requiring regions of high and low permeability.
U.S. Pat. No. 5,927,404 describes a method of using the solids-stabilized emulsion as a drive fluid to displace hydrocarbons for enhanced oil recovery. U.S. Pat. No. 5,855,243 claims a similar method of using a solids-stabilized emulsion, whose viscosity is reduced by the addition of a gas, as a drive fluid. U.S. Pat. No. 5,910,467 claims solids-stabilized emulsion described in U.S. Pat. No. 5,855,243. U.S. Pat. No. 6,068,054 describes a method for using solids-stabilized emulsion as a barrier for diverting the flow of fluids in the formation.
Use of emulsified aqueous acids for stimulating reservoirs is known in the art. For example, U.S. Pat. No. 7,303,018 describes a method of acidizing a subterranean formation where, in some embodiments, a strong acid is emulsified within an oil.
A problem that remains in solvent-dominated in situ oil recovery, is to maximize extraction of oil from oil reservoirs, including viscous oil reservoirs with maximum economy, minimizing solvent usage, minimum loss of solvent in the reservoir, and to leave minimal residual oil in the oil reservoirs. Solvent recovery remains an important component of process economics, and a need continues to exist for an improved method to minimize solvent use while maximizing oil recovery.